In receiving the satellite broadcast, the waveguide-to-coaxial converter is employed to transmit a radio wave received by a parabolic antenna to a receiving circuit. FIG. 2 illustrates an example of the conventional waveguide-to-coaxial converter. In this drawing, 1 is a waveguide, 2 is a ridge, and 3 is a coaxial line. 4 is an RF amplifier which is composed of a DC-blocking condenser 5, an amplifying transistor 6, such as GaAsFET, and the like. The ridge 2 is fixed conductingly on an inner wall surface of the waveguide 1 by screws (not shown), for example, to one end of which the coaxial line 3 is secured, the other side end of the coaxial line 3 being led out toward the RF amplifier 4. As shown in FIG. 3, the circuit of the RF amplifier 4 is made up of the condenser 5 and the amplifying transistor 6 both mounted on an insulating board 7a such as a ceramic substrate, and strip lines 7 and 8 provided on the board. The point of the coaxial line 3 is connected to the strip line 7. The condenser 5 is employed for insulating the transistor 6 from DC components from the waveguide (1) side because a bias voltage is applied to the transistor 6. For condenser 5 a printed condenser or chip condenser is employed. In operation, the incident radio wave upon the waveguide 1 is converted in wave mode by the ridge 2 provided inside the waveguide 1, the converted wave is taken out from the waveguide 1 through the coaxial line 3 and input into the RF amplifier 4.
In the foregoing conventional waveguide-to-coaxial converter, because the DC-blocking condenser 5 is built in the RF amplifier 4, the strip line 7 is provided at the input section of the RF amplifier 4 to connect the condenser 5 and the coaxial line 3. Further, this strip line 7 is connected to the ridge 2 inside the waveguide 1 through the coaxial line 3. Thus, when an impulse wave containing low frequency components, caused, for example, by lightning, comes in, it is transmitted directly to the strip line 7 and an electric discharge may possibly occur. In addition, because the amplifying transistor 6 may be a GaAsFET which is manufactured for use at very high frequencies, it may be damaged by excess voltage and, thus, there is the problem that the amplifying transistor 6 is destroyed due to the electric discharge caused by lightning if it is provided in the vicinity of the strip line 7.